The use of plasmas in material processing and/or surface treatment is widespread in industry. Plasmas may be used for all kinds of surface treatments, amongst which are cleaning and activation of surfaces, deposition such as plasma enhanced chemical vapour deposition (PECVD), etc. Plasmas are also used for improving the adhesive properties of a surface, for instance polymer surfaces. An example of this is the photo film production industry, in which plasmas are used to treat the surface of a film substrate, for instance in order to improve adhesive properties.
Plasmas are generally considered as a suitable solution for material processing, because a large flux of reactive species (radicals, ions) is generated, which may be directed to the processing zone and may be processed into the desired shape by using an appropriate electric field distribution.
It will be understood that, especially in applications directed to surface treatment, the plasma may ideally be a uniform and stable plasma. By using a uniform and stable plasma, the surface will be treated in a uniform manner as well. If for instance the adhesive properties are to be enhanced, the person skilled in the art may desire to obtain adhesive properties that are uniform across the treated surface.
In order to achieve the generation of a uniform plasma, firstly it is most important that breakdown of the plasma is uniform, and secondly that the uniformly generated plasma must be maintained as long as possible. In both of these steps major instabilities like streamer breakdown and filamentation may occur. To generate a homogeneous glow, these instabilities should be avoided.
Many efforts have been put in generating plasmas under atmospheric pressure and at low temperatures (e.g. 300 K-400 K), since this greatly enhances the number of applications while it reduces the costs of processing. Advantages of using atmospheric pressure are a larger density of reactive species than in the low pressure case, and the advantage of avoiding vacuum technology. Generation at low temperatures will make the technology applicable to the treatment of thermoplastic polymer surfaces. Another asset would be the ability to generate stable plasmas using air instead of other gasses, since air is cheap and readily available.
Generating a plasma under the above circumstances is not a straightforward technique. At atmospheric pressure, the particle density is high and as a result thereof the mean free path of reactive species is small. The processes of excitation and ionisation are restricted to a limited area, and the plasma is generated primarily in a filamentary form.
Plasmas at atmospheric pressures tend to be very unstable and will tend to develop into a spark or an arc in short time after the breakdown. Any random local increase in a current density will tend to grow rather than to be damped and the plasma will be constricted.
The effects of above-mentioned instabilities may be reduced by limiting the current density and the plasma duration by covering the electrodes with a dielectric (dielectric barrier discharge configuration, DBD). Due to charge accumulation on the surface of the dielectric, the value of the voltage applied to the plasma is reduced. When the magnitude of the voltage applied to the plasma decreases below a critical level (the cut-off voltage), the plasma can not be sustained any longer. The plasma duration will therefore be limited.
On the other hand, however, the use of “strangled” atmospheric plasmas for material processing is less efficient. Additionally, the dielectric barrier discharge may only limit the current density to a certain extent, since streamers having current densities in the range of 10 A/cm2 may still be generated on small surface areas. The dielectric barrier limits the overall current density across the surface of the electrodes used for generating the plasma, but does not prevent strong local currents due to streamer formation to occur.
It is known that the surface of the electrodes (whether or not covered by a dielectric) plays an important role in generating and maintaining the plasma. A variety of interactions between the surface, the electric field for generating the plasma and the plasma itself determine the conditions that are present in a discharge space, and therefore determine whether a generated plasma is stable and uniform or not stable and filamentary.
One of these interactions is based on generating secondary electrons at the surface of the electrode. These secondary electrons must be freed from the surface and be released in the discharge space where they may contribute to the generation of the plasma. Finding a material for which on the one hand sufficient secondary electrons are present near the surface while on the other hand these secondary electrons may be released using only limited amounts of energy is difficult. A number of materials have been proposed, often in combination with dielectric barrier discharge configuration, but finding the optimum in this balance remains a problem in the industry.